I read somewhere that the sacks that held flour and sugar were more tightly woven than the ones that held feed, so you couldn't always tell by that. If you find an edge with the holes where the thread holding the sides together was taken out, that would be a big help. I'm like you. I often find fabric labeled feed sacks and think it's as much hopeful thinking and a marketing ploy as anything. There are lots of vintage fabrics still around that weren't from feed sacks. Not much help, am I? Bottom line is, I wouldn't pay extra for feed sack that can't be authenticated, but wouldn't hesitate to take it if the price was right. Either way, nice vintage fabric is hard to find.